Shaft cone crown measurement system and methodology

ABSTRACT

Aspects include metrology methods and systems for determining characteristics of conical shaft portions, such as angle of taper and crown height. In an example, a metrology system includes a fixture for supporting a workpiece. The fixture provides for translation in a longitudinal dimension, and rotation about an axis of symmetry. The system may include a sensor mounted for scanning lines including sections of the workpiece as well as control logic for coordinating translation of the workpiece to provide for an approximately constant ratio of longitudinal translation and lines scanned during scanning operations. The system may include image logic for assembling an image from image data generated during each scanning operation, edge detection logic for detecting an edge shape in each assembled image, and slope and crown height calculation logic for calculating a slope and a crown height of each of the detected edge shapes.

BACKGROUND

1. Field

The current invention is in the field of metrology of objects.Particularly, the invention relates to metrology of characteristics ofconical shafts.

2. Related Arts

Magnetic disc drives are used for magnetically storing information. In amagnetic disc drive, a magnetic disc rotates at high speed and atransducing head “flies” over a surface of the disc. This transducinghead records information on the disc surface by impressing a magneticfield on the disc. Information is read back using the head by detectingmagnetization of the disc surface. The transducing head is movedradially across the surface of the disc so that different data trackscan be read back.

Over the years, storage density of media has tended to increase and thesize of storage systems has tended to decrease. This trend has led to aneed for greater precision, which has resulted in tighter tolerancingfor components used in disc drives. In turn, achieving tightertolerances in components requires increased precision in metrologysystems for characterizing and parameterizing those components.

Air bearing spindle motors may be designed with conical bearingelements. In such instances, the bearing elements consist of conicalshafts and corresponding sleeves. The materials used for making theseshafts and sleeves include, but are not limited to, steel with highlypolished surfaces. The angles of the cones only have a tolerance on theorder of ±0.01° and have a crown on the surface with a height of0.56±0.5 microns over the length of the surface. The crown is the amountof curvature on the outer surface of the cone. It is measured at thepoint where the distance between the surface of the cone and theimaginary line that represents the surface if there was no crown isgreatest.

Existing methods of measuring the crown include using a contact surfaceprofile measurement system. However these systems are slow, on the orderof more than 120 seconds per measurement. The systems are alsoexpensive, require a trained operator to properly align and measure thesamples. Additionally, the contact method of measuring the samples canpotentially damage the sample being measured.

Therefore, what is needed is a low-cost, accurate, and repeatablemetrology system that is accurate, fast, and able to measure cone crownheight tolerances of 0.56±0.5 microns over an 8 mm length and arepeatability (1-sigma) of 38 nm.

SUMMARY

One aspect of the invention provides a metrology system for measuringthe slope and crown height of a workpiece. The metrology system includesa fixture for holding the workpiece, a sensor that obtains line scans ofthe workpiece, a control system to coordinate the sensor and the fixtureso that the line scans can be obtained at approximately equal intervalsduring scanning, a program for assembling an image from image datagenerated during the scanning, edge detection software for detecting anedge shape in each assembled image, and calculation software forcalculating the slope and the crown height of each of the detected edgeshapes.

Another aspect of the invention provides a metrology method formeasuring the slope and crown height of a workpiece. The method includesthe steps of taking multiple line scans of a workpiece, assembling animage from the line scans, detecting an edge shape in the assembledimage, calculating the slope and the crown height of the edge shape, andobtaining both the slope and crown height of the surface of theworkpiece based on the calculated slopes and crown heights of the edgeshape.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail in the followingby way of example only and with reference to the attached drawings, inwhich:

FIG. 1 illustrates a cross-section of a portion of an exemplary discdrive having conical shaft elements;

FIG. 2 illustrates an exemplary schematic view of the shaft conemetrology system;

FIG. 3 illustrates a perspective view of an exemplary shaft conemetrology system;

FIGS. 4A-C schematically illustrate an exemplary staging system, andsources of uncertainty arising therefrom;

FIGS. 5A-B illustrate an end-on view of an exemplary fixture, anduncertainty arising therefrom;

FIG. 6 illustrates aspects of edge detection used in the exemplary shaftcone metrology system;

FIGS. 7A-D illustrate exemplary image characterization aspects; and

FIG. 8 illustrates exemplary steps of a method for shaft cone metrology.

DETAILED DESCRIPTION

The following description is presented to enable a person of ordinaryskill in the art to make and use various aspects of the inventions.Descriptions of specific materials, techniques, and applications areprovided only as examples. Various modifications to the examplesdescribed herein will be readily apparent to those skilled in the art,and the general principles defined herein may be applied to otherexamples and applications without departing from the spirit and scope ofthe inventions. For example, aspects and examples may be employed forcharacterizing and parameterizing any of a variety of objects. Forexample, aspects of shaft cone quality may also be characterized. Insome cases, shapes other than cones may also be characterized. Theexemplary system configurations, components, exemplary tolerances,design criteria, and the like provided herein are for illustratingvarious aspects and are not intended to limit the configurations,components, tolerances, and/or criteria that may be accounted for insuch metrology systems.

FIG. 1 illustrates a cross-section of a disc drive motor portion. Theportion includes a hub 10 supporting discs 12. In operation, the hub 10rotates about a fixed shaft 14. The fixed shaft 14 includes an uppershaft bearing cone 16 and a lower shaft bearing cone 18. An outersurface 34 of the upper shaft bearing cone 16 forms an upperhydrodynamic bearing region 20 with opposing upper conical bearingsleeve 28. An outer surface 32 of the lower shaft bearing cone 18 formsa lower hydrodynamic bearing region 24 with opposing lower conicalbearing sleeve 30. For proper operation, each of the shaft bearing cones16 and 18 and respectively opposing conical bearing sleeves 28 and 30should fit together. An aspect of this fit is the angle at which theshaft bearing cones 16 and 18 taper. To continue rapid and consistentincreases in disc drive performance, the taper angle of lower shaftbearing cone 18 and upper shaft bearing cone 16 should be controlled.For example, some potential designs may call for tolerances within atleast about 0.01 degrees of the engineered specification. In turn,determining whether shaft bearing cones 16 and 18 are within 0.01 degreeof specification requires an accurate metrology device and method.

FIG. 2 illustrates a schematic view of an exemplary metrology system 200for characterizing aspects of conical shaft portions (such asdetermining cone taper angles and cone crown measurements). Themetrology system 200 includes a base 202 that may be formed from graniteor another material suitable for providing support and for helpingisolate the remainder of the metrology system 200 from vibrations andother undesirable environmental influences. A stage 204 is placed uponthe base 202. The stage 204 is moveable in a longitudinal dimension, asindicated by arrows 203. The stage may be designed and secured orotherwise coupled to the base 202 such that during longitudinalmovement, the stage 204 remains substantially stationary in otherspatial dimensions. A fixture 206 is coupled with the stage 204. Thefixture 206 supports a workpiece 210 on a shaft 207 for rotation aboutan axis of symmetry of the workpiece 210 (exemplary workpiece 210 isconical and therefore has an axis of symmetry parallel with the page inFIG. 2). In the exemplary metrology system 200, the fixture 206 holdsthe workpiece 210 to maintain the axis of symmetry in a positionsubstantially parallel to the longitudinal dimension (arrows 203) inwhich the stage 204 moves. The fixture 206 preferably holds theworkpiece 210 in place using a vacuum nest.

A camera 212 is mounted such that the stage 204, in moving in thelongitudinal direction, moves the workpiece 210 across a field of viewof the camera 212. The camera 212 may be a line scan camera forobtaining an image of a single line of pixels each time the camera isoperated. An image may be assembled from a plurality of line scans. Aline scan camera may be preferable over a camera that obtains an entireimage at one time for a variety of reasons. Such reasons may include alower price per pixel, improved dynamic range of the pixel sensors, ahigher pixel fill-factor, and elimination of frame overlaps. A line scancamera may also allow obtaining a higher resolution of a total image.

Camera 212 may be analog or digital, however a digital camera bydigitizing image data closer to the sensing source of that data mayprovide a lower noise image. Camera 212 may be color (i.e., sensitive toand capturing a range of light wavelengths) or camera 212 may bemonochrome. A monochrome camera may be preferred for the exemplarymetrology system 200 because color cameras may be affected by coloraliasing in an image having sharp contrasts between portions of theimage. In the present context of a digital camera, camera 212 maygenerally viewed as a sensor that is controllable to capture image datafrom a source on command. As such, camera 212 may also operate with anoptics system.

In exemplary metrology system 200, telecentric optics system 213 isdisposed so that the camera 212 captures image data through thetelecentric optics system 213. A telecentric optics system reducesperspective error (parallax) induced by changes in the distance betweenan object from which reflected light is being sensed (i.e., aphotographed object) and the sensor. In the present case, perspectiveerror would be caused by the workpiece 210 being closer to or fartherfrom camera 212, for example during rotation of workpiece 210, as willbe further explained herein. Perspective error would cause metrologysystem 200 to be less accurate because workpiece 210 would be appear tobe differently sized in images assembled from scans taken at differentpoints of rotation.

Dashed plane 240 illustrates an approximately perpendicularcross-section (perpendicular to an axis of rotation of workpiece 210) inwhich camera 212 captures line scans in the present example. Line scanscapturing approximately perpendicular cross-sections are convenient.However, line scans having non-perpendicular cross-sections may also becaptured, so long as appropriate corrections are made to otheralgorithms and components of metrology system 200.

Exemplary metrology system 200 may also include high intensity (lowexposure) backlighting 208 that emits light for silhouetting workpiece210 for camera 212. Backlighting 208 may be comprised of light emittingdiodes. Backlighting workpiece 210 may provide for sharper definition ofthe edges of workpiece 210 by helping enhance contrast of workpiece 210edges against the backlighting. A color (wavelength) of backlighting 208may be selected based on a sensitivity spectrum of camera 212. A lowerwavelength may be preferable for reducing diffraction caused byworkpiece 210. Control system 250 may also interface with backlightcontroller 222. Backlighting 208 may also be strobed in time with thetaking of line scans by camera 212 under control of backlight controller222.

An image acquisition logic 220 interfaces with camera 212 to receiveline scan output. Image acquisition logic 220 interfaces with controlsystem 250. Image acquisition logic 220 may assemble an image from theline scans outputted by the camera 212. Image acquisition logic 220 maythen provide the assembled image to control system 250 for further imageprocessing, and the like. Image acquisition logic 220 may also provideportions of an image, or as little as a single line scan to controlsystem 250. However, as one of skill in the art would understand,control system 250, if it is to receive line scan image data must beable to respond quickly enough and/or have sufficient buffer space tostore line scan image data until that data can be processed.

Timing of when camera 212 captures a line scan should be controlled inpresent examples of metrology system 200. Image acquisition logic 220may control this timing either independently, under direction of controlsystem 250, or by some other suitable control means. The control of whencamera 212 captures a line scan may also be referred to as shuttercontrol, and a complete cycle of line scan capture may described withreference to a shutter speed. In turn, a frequency of line scan capturemay be derived from the timing of the capture, and may be affected byhow quickly the sensors of camera 212 can capture enough light togenerate an image.

Control system 250 also interfaces with stage and fixture controller224. Stage and fixture controller 224 controls longitudinal movement ofstage 204 and rotation of fixture 206. Longitudinal movement of stage204 should be coordinated with line scan capture such that a line scanis captured at approximately equal intervals of longitudinal movement,and with a frequency selected to achieve a desired fidelity during imagereconstruction. As such, control system 250 should contain logic tocoordinate stage and fixture controller 224 with image acquisition logic220. Thus, metrology system 200 may provide for communication betweenthe image acquisition logic and the stage and fixture controller logic224 for aiding in this coordination/synchronization. In other examples,control system 250 may communicate with one of image acquisition logic220 and the stage and fixture controller logic 224, and thereaftersynchronization may be accomplished without involvement of controlsystem 250.

In exemplary metrology system 200, the stage and fixture controller 224causes the stage 204 to move longitudinally through about 8 mm. Duringthat longitudinal movement, the image acquisition logic provides forcapture of approximately 8000 lines of image data. Thus, in such anexemplary metrology system 200, a line of image data is captured forapproximately each 1 μm of longitudinal movement. A more particularexample is provided with regard to FIGS. 4-8, below.

FIG. 3 illustrates a perspective view of an exemplary arrangement ofcomponents of the exemplary metrology system 200 of FIG. 2. As discussedabove, base 202 provides a stable supporting structure for portions ofmetrology system 200 described below. In this example, a camera support310 is secured to base 202 at two locations and may be formedsubstantially in a U shape, with camera 212 and associated telecentriclens 213 coupled thereto. A convenient aspect of the implementation inFIG. 3 is that stage 204 may be mounted to base 202 between the twolocations at which camera support 310 is secured to base 202. In thisaspect, stage 204 may move perpendicular to a plane of the U, whichallows workpiece 210 to be easily translated under telecentric lens 213for line scan captures by camera 212. As illustrated, fixture 206 ismounted to stage 204 and provides for an ability to rotate workpiece 210for obtaining line scans at various different rotation positions.Rotating workpiece 210 may aid in reducing errors caused byimperfections in the workpiece, such as surface blemishes,eccentricities, and roundness errors. Rotating workpiece 210 may alsoaid in reducing errors from other components of metrology system 210,such as runout of fixture 206.

Stage 204 may be coupled to base 202 through stage guide 325. Stageguide 325 may provide railing portions to aid in guiding stage 204.Stage 204 may be an air bearing stage, as further described below.

In other exemplary metrology systems, instead of, or additionally to,rotation of workpiece 210, camera 212 may be rotated or otherwise movedto obtain line scans from different points with respect to the workpiece210. Exemplary aspects and examples should not be construed to belimited either implicitly or explicitly to rotating only workpiece 210for obtaining images of different portions of the conical surface ofworkpiece 210.

In other examples, a full-frame imaging sensor may be operated in a linescanning mode, or an imaging sensor may be mounted on a movable supportfor providing line scanning functionality.

Above exemplary systems illustrated and described scanning“longitudinally” which was for example identified by directional arrows203 in relation to workpiece 210 (FIG. 2). Other examples of relativetranslation of a workpiece and an image sensor are contemplated. Forexample, relative translation may be such that line scans may be takenparallel to a direction of rotation of workpiece 210 (as opposed toperpendicularly as in FIGS. 2 and 3). And if line scans are obtained bysome other mechanism than scanning with a line scan sensor, for exampleby sequencing a sensor having multiple lines, then translation examplesherein may be adapted accordingly. Thus, the exemplary line scanningfunctionality and apparatuses should not be considered as a sole way ofscanning lines to include cross-sections of an object of metrology.

The above examples included references to, for example, control system250, stage and fixture controller 224, image acquisition logic 220, andother logical and control functions. These example logical functions andcontrol features may implemented in any of a variety of ways, includingby dedicated hardware for each function such as by application specificintegrated circuits or programmable custom hardware (e.g., FPGA), byintegrating various functions into dedicated hardware, by providingprogram code implementing one or more described functions in one or moregeneral purpose microprocessors, by dividing some portions of thefunctions into mechanical control and others into electronic and/orsoftware control. Any combination of the above variations may also beeffected.

Further exemplary aspects of metrology system 200 are discussed withregard to FIGS. 4A-C. Each of FIGS. 4A-C illustrate a different aspectof error induced by stage 204. FIG. 4A illustrates aspects of yaw andlinear error. An ideal stage 204 would linearly translate in onedirection (dimension), but would not tilt, lurch, pitch, roll, orotherwise move in any other direction (dimension). However, stage 204may exhibit a variety of errors in actual usage. One error is linearpositioning error represented by double ended arrow 415. As describedabove, stage 204 translates workpiece 210 under camera 212. In manyaspects, this translation should be controlled and repeatable. However,there is some linear error in the stage position, as the stage mayeither be slightly further along or lagging where it should be at anygiven time.

Other errors in the stage 204 may include straightness and flatness ofmovement errors (Δstraightness), each of which are non-rotationalmovements in directions other than the direction of movement.Straightness error refers to movements not precisely parallel to anintended direction of movement.

Yaw errors (ε_(yaw)), illustrated by curved arrow 420 in FIG. 4A arerotational errors about an axis perpendicular to the direction oftravel, and not in the horizontal plane of stage 204. FIG. 4Billustrates direction of travel arrows 203, and curved arrow 425 thatrepresents pitch errors (εpitch). Pitch errors are rotational movementsabout an axis in the horizontal plane of stage 204, but perpendicular todirection of travel arrows 203. FIG. 4C illustrates an end on view ofstage 204 (i.e., stage 204 is traveling toward/away from the viewer),and curved arrow 430 which represents roll errors (ε_(roll)). Rollerrors are rotational errors about an axis parallel to the direction oftravel indicated by direction of travel arrows 203.

Other sources of error may include scale error (δ_(scale)) introduced byimprecision in the encoder and motor components of stage 204. All ofthese errors may be objects of minimization in exemplary aspects. An airbearing stage may be used for aiding in such minimization, with anexemplary air bearing stage being the FiberGlide 1000 from Aerotech.

Effects of the above described errors on accuracy and repeatability ofmetrology system 200 may be calculated and comprehended in designingmetrology system 200. Error in the direction of motion (illustrated as203 in FIGS. 2 and 4B) and perpendicular to the direction of motion inthe horizontal plane of stage 204 can be calculated as demonstratedrespectively in equations 1 and 2 below.

Δy=(δ_(scale)+ε_(pitch) x·z+ε _(yaw) x·x)   Equation 1

Δx=(δ_(straightness)+ε_(roll) x·z)   Equation 2

A measurement taken with metrology system 200 may have a differentsensitivity to some of the above sources of error. For example, Δy isdirectly impacted by the length of the field of view perpendicular tostage motion (x) as this length increases the effect of ε_(yaw)Likewise, both Δy and Δx are directly sensitive to distance betweenstage 204 and a point on workpiece 210 being scanned or otherwiseimaged, as this distance increases the effect of both ε_(pitch) andε_(roll).

FIGS. 5A-B illustrate that rotation by shaft 207 of fixture 206 maycause inaccuracy in metrology system 200. FIG. 5A illustrates that shaft207 rotates on bearing system 515 that is disposed between shaft 207 andouter sleeve 510. Because a predominantly circular bearing system isoften not strictly circular, bearing systems often have eccentricitywhich is illustrated by double ended arrow 520 between outer sleeve 510and shaft 207. This error effect is known as circular runout. Exemplaryaspects use telecentric optics system 213 for reducing errors due tothis error effect.

FIG. 5B illustrates that another source of inaccuracy may be angularrunout 530 (wobble), illustrated as producing arc 530 by a central axisof the workpiece during rotation. The arc is illustrated as two arrowspointing towards the viewer illustrating two different directions inwhich the rotational axis could point during rotation, if there were awobble. Smaller wobble would cause arc 530 to be smaller. Angular runout530 causes workpiece 210 to appear shorter, and hence the cone anglelarger. Thus, angular runout 530 should be considered in design ofexemplary metrology system 200.

FIG. 6 schematically illustrates how effects of angular runout 530 canbe considered in design of exemplary metrology system 200. Solid coneshape 602 illustrates an outline of workpiece 210 captured at a givenangle of rotation of shaft 207. Dashed cone shape 604 illustrates anoutline of workpiece 210 at a different angle of rotation of shaft 207.As illustrated, due to angular runout 530, a length 610 of the workpiece210 for shape 602 appears longer than a length 605 of the workpiece forshape 604. These lengths in turn affect respective apparent cone angles625 and 620. Thus, error due to angular runout 530 may be calculated asshown below, where a maximum runout is defined by N and L is a nominallength of the workpiece 210.

$\begin{matrix}{{error}_{future} = {{\beta - \alpha} = {{\tan^{- 1}( \frac{D}{2\; L} )} - {\tan^{- 1}( \frac{D}{2\; M} )}}}} & {{Equation}\mspace{14mu} 3} \\{M = {L\; \cos \; \theta}} & {{Equation}\mspace{14mu} 4} \\{\theta = {\sin^{- 1}( {N/L} )}} & {{Equation}\mspace{14mu} 5}\end{matrix}$

This result may be probabilistically accounted for by assuming a normaldistribution and a 95% confidence level, with a coverage factor of 2,which would provide a standard error of fixture equal to one half of thecalculated error.

As described with respect to FIGS. 4A-C, 5A-B, and 6, various errors andinaccuracies in components used to construct exemplary metrology system200 may be considered so that the system performs as expected and withintolerances. Such components and calculations and errors relating theretoare exemplary, and may be adapted by substitution of differingcomponents as would be understood by one of ordinary skill in the art,including designing systems having other accuracies. As componenttolerances improve, it would be expected that tolerances of othercomponents may be relaxed, and that an overall accuracy of a system mayimprove.

Now turning to FIGS. 7A-C, aspects relating to extracting (detecting)edges of workpiece 210 from captured images are discussed. As usedherein extracting may include any operation for producing datadescribing positional and orientational aspects of such edges. In FIGS.7A-C, captured image 706 illustrates workpiece image 705 againstbackground 707. In an example, pattern recognition logic may be used foridentifying a feature of workpiece image 705 (signifying a feature ofworkpiece 210) as displayed in captured image 706. For example, patternrecognition logic may identify a corner area 708 of workpiece 210. Suchpattern recognition logic may be implemented in control system 250.After identifying the feature of workpiece 210, a coordinate system maybe overlayed on the workpiece image 705 with reference to the feature;this coordinate system is illustrated for convenience by site mark 710in FIG. 7A.

After identifying the feature and setting up the coordinate system,FIGS. 7B and 7C illustrate providing search areas 715 and 720respectively for the left and right edges of workpiece image 705. Searchareas 715, 720 may be located with reference to the coordinate system.By locating the search areas 715, 720 with reference to the coordinatesystem, the search areas move with the identified feature of theworkpiece. For example, in a production metrology situation, workpiecesmay be loaded on metrology apparatus 200 so that all the workpieces arenot in the same physical location during scanning operations. Such asituation would cause various workpiece images 705 (images of differentworkpieces or rotationally varied images of the same workpiece) toappear in different locations within captured image 706. Because searchareas 715, 720 move with reference to the identified feature, the searchareas remain within the relevant areas of workpiece image 705 (i.e.,remain over edges to be identified). This aspect may increaserepeatability by compensating for the exemplary situation describedabove and other sources of variation during use of metrology system 200.

FIG. 7D illustrates a portion of image 705 wherein a crown aspect of aworkpiece is emphasized. Dashed line 756 illustrates where a surface ofthe workpiece would generally occur, if the workpiece were uncrowned. Acrowned surface of the workpiece is identified as 755, and as shown hereis slightly concave from line 756. A maximal distance between 755 and756 can be considered a crown height.

FIG. 8 illustrates steps of an exemplary metrology method. In 802, aworkpiece (e.g., workpiece 210 of FIG. 2) is loaded into the fixture206, such as via vacuum nesting. In 804, the stage movement is commencedby the stage controller 224 as controlled by control system 250. At 806,the camera 212 captures lines at intervals determined based on a desiredresolution, such as the resolution arrived at by the above exemplarycalculations for exemplary metrology system 200. 806 may be viewed as aniterative process including a decision block 808 for determining whethera given image scan has been completed (e.g., completed such as to formcaptured image 706). In practice, any number of different methods andmeans can be used for determining whether all lines for assembling animage have been captured, such as timers, counters, image featuredetecting algorithms, sensors for detecting a position of stage 204,sensing how much memory has been used in a buffer storing image data,and the like.

Following a determination that all the data for assembling an image hasbeen captured, the image is assembled at 810 and may be enhanced at 812.Image enhancement may allow for extraction of better lines in latersteps. Image enhancement may include contrast modification, dilation,erosion, and other enhancements known to those of skill in the art ofimage processing. Following image enhancement, edges may be extractedfrom the image at 814 (e.g., edges of workpiece image 706 of FIGS.7A-C). The characteristics (such as slope and crown measurement) ofextracted edges may be calculated at 816. In an example, edges extractedfrom the image are expressed as a series of coordinate points (e.g., aseries of pixel data), which may be stored in an array or other datastructure.

For determining the slope of the extracted edges, a curve fittingalgorithm (e.g., a least square means algorithm) may be used to fit aline to that series of coordinate points. After fitting, the curve maybe expressed by an equation that expresses slope of the edge to whichthe curve was fitted. This slope expresses the angle of taper of theworkpiece 210. Other methods for determining a slope of an extractededge may include calculating a rise over run based on sampled pixeldata, and any other method known by those of ordinary skill in the art.

For determining the crown height measurement of the extracted edges,residual higher order terms may be extracted after the determination ofthe slope of the extracted edges. These residual terms may then bepassed through a Median Filter (i.e. a non-linear digital filter) toreduce the noise in the data. The filtered data (Yi) is determined bythe equation:

Yi=Median(Ei)i=0, 1 . . . n−1   Equation 6

Where Ei is the subset of the data measured in sequence where Ei(x_(i−r). . . x_(i−1), x_(i), x_(i+1), . . . , x_(i+r)) and where each xrepresents a data acquisition point.

In an alternate embodiment, the data is fed through a DigitizationNoise-Image Averaging Filter. Image averaging is a digital imageprocessing technique that is often employed to enhance video images thathave been corrupted by random noise. In order to reduce the algorithmmemory requirements, it is possible to compute a Running Average of acontinuous image stream by applying the following equation:

A(N, x, y)=[I(x, y)+(N−1)·A(N−1, x, y)]/N   Equation 7

Where N is the number of image frames to be computed, A(N, x, y) is theintensity values of pixels located at coordinates (x, y), I(x, y)represents the intensity value of a pixel at the location (x, y) in theimage frame, and A(N−1, x, y) represents the average pixel intensityvalue from the previous N−1 image frames.

Regardless of the filtering method used, a second order polynomial maythen be fit to the filtered data. Based on the second order polynomial,the crown height measurement can be computed based on a peak to peakmeasurement. Once the second order polynomial is determined, anR-squared (Rsq) value for each edge can be calculated to determine thegoodness of fit of the second order polynomial to the crown asdetermined by the filtered data. Rsq can be determined using thefollowing equation:

Rsq=1−(SSE/SST)   Equation 8

Where SSE is the sum squares error and SST is the sum squares total. TheRsq values may then be used to accept or reject the crown measurementdata.

After calculation of the characteristics of the extracted edges, adetermination is made at 818 as to whether more images should becaptured such that more edges can be extracted and slopes and/or crownheights obtained therefrom. By averaging a certain number of edge slopesand/or crown heights, better accuracy may be obtainable since workpiecesbeing measured may exhibit local abnormalities and imperfections thatinduce measurement inaccuracies not easily separable for inaccuraciescaused by the metrology apparatus. This determination 818 can includecomparing the number of edge slopes and/or crown heights calculated witha predetermined number of edge slopes and/or crown heights, or a numberof images with a predetermined number of images, since by analysis itcan often be determined how many different scans will be required toobtain a required measurement accuracy, based on expected uniformity ofthe workpiece. Other determinations can include comparing resultsobtained between slopes and/or crown heights of edges extracted fromdifferent images to detect whether variation appears to be present amongthe slopes and/or crown heights, or whether all the slopes and/or crownheights are within an acceptable tolerance.

In cases where more images should be extracted, the workpiece may berotated by rotation of the shaft 207 through some arc. As describedabove, camera 212 may be rotated instead of or additionally to rotationof workpiece 210. The method then loops back to 804 for beginning stagemovement once again. If no more images/edge slopes are required thenthose characteristics calculated are averaged or otherwise combined at822, and the averaged characteristics are outputted at 824. Thoseoutputted characteristics are compared with a specification at 826, andif all of the characteristics are within the specification, then theworkpiece is allocated for use in a disc drive motor or some othersuitable mechanism at 828. And if the workpiece is not within thespecification, then the workpiece may be discarded and another workpieceloaded by returning to 802, or the workpiece may be subjected to anothermetrology run.

Such aspects as described above are merely exemplary, and can bemodified, extended, and/or redacted as required for a particularapplication. For example, a number of images can be captured beforeedges are extracted from images and characteristics calculated. Slopesand/or crown heights can be averaged or combined in any number of ways.In some aspects, the extracted edges themselves may be averaged (e.g.,by averaging pixel or coordinate level data) and a slope and/or crownheight extracted from the average of the edges. In other examples,various edge averages may be created, a slope and/or crown heightextracted for each created edge average, and slopes and/or crown heightsextracted from those edge averages.

This description is exemplary and it will be apparent to those ofordinary skill in the art that numerous modifications and variations arepossible. For example, various exemplary methods and systems describedherein may be used alone or in combination with various additionalmetrology systems and other systems for determining suitability of aworkpiece under a given specification. Additionally, particular exampleshave been discussed and how these examples are thought to addresscertain disadvantages in related art. This discussion is not meant,however, to restrict the various examples to methods and/or systems thatactually address or solve the disadvantages.

1. A metrology system, comprising: a fixture for supporting a workpiece;a sensor operable for obtaining line scans which include sections of theworkpiece; control logic for coordinating the sensor and the fixture tocause line scans comprising image data to be obtained at approximatelyequal intervals during scanning operations; image logic for assemblingan image from the image data generated during each scanning operation;edge detection logic for detecting at least one edge shape in eachassembled image; and calculation logic for calculating a slope and acrown height of the at least one detected edge shape.
 2. The metrologysystem of claim 1, wherein the fixture supports the workpiece forrelative translation with respect to at least one of the sensor and afixed position of the sensor.
 3. The metrology system of claim 1,wherein the fixture and the sensor are relatively disposed to providefor capture of rotationally varied views of the workpiece.
 4. Themetrology system of claim 1, wherein the control logic further providesfor rotation of the workpiece after a completion of each scanningoperation and before a commencement of each subsequent scanningoperation, thereby resulting in at least two images of each edge shape.5. The metrology system of claim 4, wherein the workpiece is a maleconical portion, the edges are representative of a slope of the maleconical portion, the calculated crown heights for respective edges arerelated to the crown height of the conical portion, and the calculationlogic provides for averaging of the calculated crown heights.
 6. Themetrology system of claim 1, wherein the sensor includes a line scancamera portion operable to capture line scans through a telecentriclens.
 7. The metrology system of claim 1, wherein at least two edgeshapes are detected in each assembled image and the calculated crownheight of the at least two edge shapes is averaged.
 8. The metrologysystem of claim 1, wherein the edge detection logic is provided with anidentified subregion of each assembled image for each of the at leastone edge shape and crown height to be detected in that assembled image,each identified subregion associated with a coordinate system associatedwith an imaged feature of the workpiece detected in that assembledimage.
 9. The metrology system of claim 1, further comprising: a LEDbacklight for silhouetting the workpiece; and a light intensitycontroller for controlling the LED backlight.
 10. The metrology systemof claim 1, wherein the fixture includes a vacuum nest.
 11. Themetrology system of claim 1, wherein the calculation logic uses a datafilter and a second order polynomial to determine the crown height. 12.The metrology system of claim 1, wherein an R-squared value iscalculated to determine a goodness of fit of the second orderpolynomial.
 13. A metrology method, comprising: taking a plurality ofline scans of a workpiece; assembling an image from the plurality ofline scans; detecting at least one edge shape in the assembled image;calculating a slope and a crown height of the at least one edge shape;obtaining a slope of a surface of the workpiece based on the calculatedslopes of the at least one edge shape; and obtaining a crown height of asurface of the workpiece based on the calculated crown height of the atleast one edge shape.
 14. The metrology method of claim 13, furthercomprising translating the workpiece with respect to a sensor taking theplurality of line scans, the translation providing for approximatelyequal intervals or as commanded by the encoder between each of theplurality of line scans.
 15. The metrology method of claim 14, furthercomprising: repeatedly rotating the workpiece through portions of acomplete revolution, and, after each rotation, repeating the steps oftranslating, taking a plurality of line scans, assembling an image,detecting at least one edge shape, and calculating a slope; obtainingthe slope of the surface of the workpiece by averaging the calculatedslopes; and obtaining the crown height of the surface of the workpieceby averaging the calculated crown heights.
 16. The metrology method ofclaim 13, further comprising selecting a search region in which the atleast one edge shape is to be detected, the search region associatedwith a coordinate system mapped to an imaged feature identified in theassembled image.
 17. The metrology method of claim 13, furthercomprising determining if the slope and the crown height of the surfaceare within a specification and if so then allocating the workpiece forassembly into a disc drive motor.
 18. The metrology method of claim 13,further comprising filtering the data and fitting a second orderpolynomial to the filtered data to determine the crown height.
 19. Themetrology method of claim 18, further comprising calculating anR-squared value and determining the goodness of fit of the second orderpolynomial.
 20. A metrology system, comprising: a stage translatable inone dimension while remaining substantially constant in two otherdimensions; a fixture coupled with the stage, the fixture for holdingand providing for controllable rotation of a workpiece; a sensorarranged for capturing line scans of the workpiece through a telecentriclens; a backlight for silhouetting the workpiece for the sensor; logicfor maintaining an approximately constant ratio between longitudinaltranslation of the stage and timing of line scan captures; an imageassembler for receiving line scan captures and assembling an imagetherefrom; logic for detecting one or more edge shapes in the assembledimage; and logic for determining an angle and a crown height of asurface of the workpiece from the one or more edge shapes.